1. Field of the Invention
The present invention relates to a digital radio telephone apparatus used in a cellular radio system, and more particularly, to a digital radio telephone apparatus having an equalizer selectively employed for compensating for signal distortion caused by the multipath fading.
2. Description of the Related Art
As the number of subscribers in cellular radio systems increase, digital cellular systems are desirable in addition to the presently available analog cellular systems. In the digital cellular system, the speech signals may be encoded into digital signals and transmitted at a more efficient transmission rate. Due to the mobility of the mobile unit to be served by the system and geographical conditions, however, the transmitted signals may be degraded when mobile units receive the signals. This problem is called a multipath fading problem. To overcome this problem, an equalizer may be employed in the apparatus. If the equalizer is employed in the apparatus, wave distortion caused by interference between signals directly transmitted and delay signals indirectly transmitted may be compensated for by the equalizer.
However, when the mobile unit moves near the base station, the fading problem is not so serious. Rather, near the base station, the bit error rate measured when the equalizer is employed may be higher than the bit error rate measured when the equalizer is not employed. Still, if the equalizer is formed in a chip of CMOS (Complementary Metal Oxide Semiconductor), electrical power is greatly consumed in the chip while the chip is in an operational state. Further, while signals applied to the equalizer fluctuate over a wide frequency range, the equalizer does not operate accurately and thereby it takes time for the mobile unit to establish a radio link with a base station.